Photophysical properties of biphotochromic dihydroindolizines.: Ring-opening into extended bis-betaines

New photochromic spirodihydroindolizines (DHI) that are mono-(1a-e) or biphotochromic (3a-e) have been prepared. The monophotochromic DHIs 1d and 1e contain sterically highly demanding substituents. Photophysical studies, both static and Lime-resolved, were carried out with 1 and 3. Monophotochromic DHIs 1a-e show fluorescence and phosphorescence. Temperature decrease produces a hypsochromic shift in the fluorescence spectra. Time-resolved studies demonstrate that the photochromism is based on a one-photon electrocyclic photoreaction of 1 to the colored betaine. Only one transient of 0.6-6 mu s lifetime could be detected for the transformation of 1a-e. For the bichromophoric DHIs 3a-e, two different transients were recorded with lifetimes of 1 and 45-680 mu s. It was proved that the ring opening 1 --> 4 (as well as 3 --> 9) results from an excited singlet species, as demonstrated by the absence of an oxygen effect and by sensitization experiments. Viscosity effects underline the conformational transformations involved in the sequence 1 --> T --> 4 and 3 --> T-1 --> T-2 --> 9 to occur in distinct steps, thus showing the detailed reaction paths of photochromic molecules 1 and 3.